Very cool.
Mine safety folks might like that.
Jim AA5BW
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Paul Nicholson
Sent: Sunday, January 8, 2017 6:18 PM
To: [email protected]
Subject: LF: Wideband EbNaut with prefix?
A decode from a near field test at VLF:
Rank 6938 Eb/N0 -0.4 dB 8K19A 60 chars
The interesting thing is, the symbol period is 6.25 milliseconds
(3152 symbols for 19.7 second total duration) and the message was sent by
magnetic coupling between two undisciplined soundcards. Received against the
natural VLF background.
The message was also 'unannounced'.
EbNaut here is sending a coded prefix. The decoder monitors a band and
searches for prefixes, then measures the frequency and timing accurately enough
to decode the following message coherently.
It relies on the undisciplined soundcard clock or RX LO being stable over the
message duration, say up to a minute or so.
The overhead of the prefix is at the moment 1.25dB, hoping to bring that down a
bit. A delicate matter of balancing the prefix energy and message energy. No
point in a detectable prefix if the message wont decode, or vice versa.
In principle then, with a small Eb/N0 overhead, the decoder can take a live
untimed I/Q stream from SpecLab or your favourite SDR and monitor a band a few
Hz wide. The only thing you have to know in advance is the symbol rate. Coding
in the prefix supplies the rest.
No good of course for slow narrow band, but there might be applications for
short duration wide band messages coherently detected and decoded with signals
a dB or so from the limit.
--
Paul Nicholson
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